1. Field of the Invention
The present invention teaches an improved process for clean-up of waste waters containing organic contaminants. In addition, the process is useful for breaking or resolving oil-in-water emulsions, such as those produced during oil recovery operations. Both the extent of clean-up based on residual oil left in the waste water and the efficiency of clean up based on the treat rate of additive have been found to be significantly improved employing the novel process of this invention. The novel process involves the use of a low level of a water dispersible terpolymer consisting of a water soluble nonionic monomer, such as acrylamide, a cationic monomer, such as 3-acrylamidopropyltrimethylammonium chloride, and a hydrophobic monomer, such as alkyl(meth)acrylamide or alkyl(meth)acrylate. These terpolymers are effective for the clean-up of waste waters containing organic contaminants and for the breaking of oil-in-water produced emulsions.
2. Description of the Prior Art
The production of waste water clean enough for safe disposal continues to be a problem, especially when oil is emulsified in the primary waste water. In oil production, especially where high levels of water flooding or steam flooding are being practiced, oil-in-water emulsions are generated. These water continuous emulsions are generally referred to as reverse emulsions since the more usual oil field produced fluid is a water-in-oil emulsion. Reverse emulsions are difficult to break and generally result in large volumes of oil contaminated waste water that must be treated for removal of oil and solids before disposal by either release to surface streams, injection or use as boiler water makeup for steam generation. Other oil-in-water emulsions of concern in the waste water treating area are those produced as a result of steel mill and metal working operations, food processing, refinery and chemical plant operation, cooling water blow-down, bitumen extraction from tar sands and shale oil operations, rain water run-off and a host of others. These emulsions all have in common the fact that the oil or organic phase is insoluble in the water continuous phase. The amount of oil dispersed in these water continuous emulsions varies from a few to several hundred parts per million in waste waters to several percent (5 to 25% or more) in fluids right out of the wellhead.
The oil is generally well dispersed in the water phase as very small droplets that are stabilized as a result of the presence of natural surfactants. The stability of these oil-in-water emulsions generally results from either a negative charge imparted to the droplets by these surfactants or from steric stabilization caused by surfactants, or by the shear which the fluid experiences during production, which causes the generation of smaller and more stable droplets, or from several other sources. As the density of the organic phase becomes higher, approaching that of the water phase, further stability is imparted to the emulsion. The presence of fine solids, such as clays, sands, corrosion products, etc., in the fluids add to the stability of these oil-in-water emulsions. The stability of these oil-in-water emulsions must be overcome if the oil is to be removed from the water before disposal or in order to separate the oil from the water in the produced fluids in the case of oil production in water floods and steam floods.
Current practice is to attempt to break waste water oil-in-water emulsions using various mechanical techniques, such as API separators, induced air flotation (IAF), dissolved air flotation (DAF), or other methods. In the API separator method time is allowed for the oil droplets to come together and float to the surface or to settle to the bottom if it is adsorbed on solids. In many cases just a large tank is used to provide a long holding time for a fluid under essentially low flow or quiescent conditions. The expectation is that the long residence time will permit coalescence and rising of the oil droplets. In the flotation methods air or another gas, such as natural gas or produced gases in the oil field where the waste water is generated, is used to form very small gas bubbles under the surface of the waste water which float to the surface. As the gas bubbles rise to the surface the oil droplets attach themselves and rise to the surface with them, concentrating at the surface wher the oil can be removed.
Various chemicals, surfactants and polymers are generally applied to these waters to enhance the separation of oil and water using the above methods and, in some cases, are required if the method is to work at all. These chemicals are used to aid in foam generation in flotation. In addition they may be used to cause oil droplet surface charge neutralization, which results in destabilization of the oil-in-water emulsion. This destabilization results in agglomeration of the oil droplets, floc formation, and possibly several other beneficial effects. While the use of such chemicals generally enhances the separation of oil from oil-in-water emulsions, there remains significant room for improvement. The type of water soluble polymers currently used are generally acrylamide copolymers or melamine/formaldehyde polymers, or others. For example, Bolhofner in U.S. Pat. No. 4,472,284 describes the treatment of water containing fats, oils and greases using a melamine-formaldehyde condensation product, along or in combination with a polyacrylamide. Rather high polymer concentrations are needed and a two polymer system can present handling difficulties during field operations. Another approach to the treatment of waste water involves the use of water insoluble polymeric adsorbents, as described by Renner in U.S. Pat. No. 3,716,483, or Takegani, et al. in U.S. Pat. No. 4,081,403. These processes for treating waste water are costly and cannot achieve the degree of cleanup of the present invention. Copolymers of acrylamide with various cationic monomers of various comonomer ratios have been used. Some of the cationic monomers that have been used are: methylacrylamidoalkyltrimethylammonium salts or similar acrylate esters, diallyl dialkyl ammonium salts, as described by Booth and Linke in U.S. Pat. No. 3,147,218, salts of dimethylaminoethylmethacrylate and the like. Buris, et al., U.S. Pat. No. 4,224,150, describe a process for clarifying aqueous systems employing quaternary ammonium adducts of polymerizable tertiary ammonium salts and acrylamide. These polymers are generally available as high molecular weight materials, either in aqueous solution, as emulsions of various types, or in solid form, which requires dissolution before use.
Typically, treatment polymers are mixed into the waste water using various levels of shear. The water is then allowed to stand in a quiescent state for several hours or days until the oil flocculates and floats to the surface, where it is skimmed off. The water phase containing any residual oil is removed from beneath the surface and is either further treated, reused or is disposed of. As an alternative to the above quiescent aging method, the polymer-treated water can be sent to a gas flotation device for oil removal as described above.
Reverse emulsions produced in the wellhead in steam floods or in water floods are generally treated with surfactants and polymers in a similar way. However, flotation equipment is not generally used on the wellhead fluid, but is used on the water, which we have called waste water, broken out of the produced emulsion. A more typical practice would be the use of settling tanks, which hold the produced fluid in a quiescent state at elevated temperature for a period of time. This equipment may vary from a simple tank to devices which add heat and make use of electrical fields to accelerate the breaking of emulsions.
Improved methods for resolving oil-in-water emulsions, such as those produced at the wellhead, would find significant utility and environmental benefit. For example, if the level of residual oil remaining in the treated water could be reduced or the speed of oil removal increased, then a more economical treating operation would result. These improvements in treating method would provide both an economic and environmental benefit. The present invention describes an improved process for treating oil-in-water emulsions.